Isn't it funny how when we make a big deal out of something, the universe will sometimes show us that it was not such a big deal after all. Sometimes in rather round-about ways that allow you compare to much worse scenarios.
So, let's say the silicone nightmare really wasn't so bad.
Here is the update:
After talking with Bub about the situation, he informed me of a temperature mix valve in the shower that will prevent water from flowing if the hot water line is not on.
Since I had not yet actually tried the heater, (I've been delaying turning it on because... well, let's just say I had a bad feeling about it.) it became the obvious next step.
So I came home and got things lined up. I opened the cut off valves for the sinks and turned on the hot water flow. I then climbed up to the loft and opened the valve to let the cold water into the heater. I heard a lovely rushing sound, and was pretty pleased with myself as I waited for the tank to fill.
Just as I was about to climb behind it and plug it in to get ready to turn it on, POP, WOOSH... i had NO idea what opened, but SOMETHING was spraying water out from what seemed to be the back side of the unit, which was stuck up against the wall in an unaccessible way due to the stabilizer frame bub built to hold the unit in place.
After a complete freak out complete with screaming, swears, and crying, I managed to get ahold of myself a bit. Moth came over and helped me deal with it...we soaked up whatever we could with towels, and messed around with it some more to try to figure out what happened.
Finally, I took the drill out and took the framing off so we could slide it away from the wall a little.
What I discovered was a second pressure relief valve opening, much like the one on the top with the pressure relief line installed in it.
I was pretty confused at first... I didn't understand why that plug popped when the top relief spigot never activated at all. I STILL don't entirely understand how that relief valve works.
After leaving a crazy voicemail, I heard back from Bub, who mentioned a small brass screw-plug thing that was in the "spare parts" box. He didn't know if this was what I needed, but thought maybe it was supposed to go in that second opening, whichever one you don't use for the relief valve.
This morning, I climbed up there and pryed out the plastic "cap" in the relief valve, and screwed in the brass plug. It seems to be water-tight, and must be it's designed purpose.
I don't know why the plumber didn't know about the need to close up this second valve opening. I'm kind of frustrated that he didn't. Now I am worried about how much water poured down the back side of the shower area with the batten insulation and everything. God knows, really.
I gave up entirely on this project last night. That is how much the straw broke the camels back. I literally said, "I'll throw the whole fucking thing in the trash!"
I am feeling a little more balanced about the situation now, but I am still entirely terrified that we will develop a mold problem behind and around the shower. Pretty much the worst possible place for a whole bunch of water to fall, after we urethaned and I meticulously sealed in the entire shower, god-knows-how-many-gallons falls BEHIND the shower.
Mold is pretty much my worst enemy, and if we end up with a mold problem I will probably cry like a little girl.
Well, enjoy your relatively predictable reality!